Isolator for power system instrument

Claim 1

An instrument isolator for isolating a voltage signal at a floating potential, applied to a pair of input terminals of one circuit, from an electrical instrument at ground potential adapted to receive and process said voltage signal and having a display consisting of a periodically repeated electronic image with a time interval between images, initiated by a cyclically triggered signal, comprising: a transformer with a center-tapped primary winding, a center-tapped secondary winding and a core magnetically coupling the primary and secondary windings; terminal means coupling said voltage signal directly to the center tap of said primary winding; a modulator circuit connected to said primary winding for alternately applying said voltage signal to opposite sections of said primary winding and including self-contained photoelectric switching elements for alternately switching said modulator circuit at a suitable modulating frequency; a demodulator circuit connected to said secondary winding for alternately connecting each end of the secondary winding to a common return point to demodulate the output signal from the center tap of the secondary winding and including second switching elements in phase with said photoelectric switching elements for alternately switching said demodulator circuit; and control means for operating said photoelectric switching elements and said second switching elements to provide a time interval between alternating pulses of said modulating frequency so as to avoid overlap between half cycles; said control means being operatively connected to said electrical instrument whereby said trigger signal synchronizes said time interval between images with said time interval between alternating pulses of said modulating frequency, whereby the demodulator output signal is representative of said input voltage signal and electrically isolated from said input terminals and said time interval between pulses is not visible on said instrument display.